The present disclosure relates to metamaterials, such as terahertz metamaterials. More particularly, it relates to high quality-factor terahertz metamaterials designed as a micro-scale closed ring resonators, and methods for fabricating the same.
Metamaterials (MMs) are artificial materials that can create unique physical and optical properties unseen in natural materials that makes them suitable for various applications in sensors, optical devices, plasmonic devices, etc. For example, the resonant behavior of a MM is dependent upon its surroundings. The chemical and physical property changes of the media in which the MM is located can affect both the frequency and the magnitude of the resonant peaks. Thus, MMs, such as terahertz metamaterials (THz MMs), are good candidates as sensors for the detection of chemicals and biomaterials, temperature, strain, alignment, and position. THz MMs can also be used as frequency-agile devices by adding a dielectric material around the MMs.
MM can usually be achieved by engineering metallic components that are smaller than the wavelength of the incident electromagnetic wave to form periodic patterns or arrays. The special arrangement of the subwavelength metallic components can be used to manipulate electromagnetic waves in such a manner that the incident electromagnetic can be absorbed, transmitted, enhanced, bended and shifted. The ability to absorb and transmit electromagnetic waves at different frequencies gives the MM the potential to be used as microwave and optical absorbers, modulators, and filters. The MM can also be designed to enhance the electromagnetic signals, resulting in a high-gain antenna. By bending and controlling the path of light and electromagnetic waves inside and around the MM, perfect lenses and cloaking systems can also be achieved.
A common MM design is as a planar, film-based subwavelength resonator that allows electromagnetic wave coupling within the structure, resulting in the storage of energy inside the resonator. With MM sensor applications, the sensing resolution and frequency selectivity of the MMs depends on their quality factors (Q-factors) because high Q-factors mean the MMs have sharp resonant responses, allowing detection of small frequency shifts induced by substances around the MMs. The Q-factor is defined as the energy stored in the MM over the energy dissipated to its surroundings. A high Q-factor of the MM means that the MM has a high signal-to-noise ratio, which leads to high sensitivity and selectivity as MM sensors and MM frequency tunable devices. Even though THz MMs show great promise for sensing and tunable devices, their relatively low Q-factors (typically below 10 for a film-based single-ring resonator MM) as compared to micro and nanoscale mechanical resonators (typically between 104 and 107) impose limitations on their sensitivity. Further, the low Q-factor causes the resonant peak to be wide, which is not suitable for applications such as narrow bandwidth filters and modulators.
One of the approaches to increase the Q-factor of MMs is to reduce the energy losses of MMs and substrates by optimizing the material properties and structures of the MMs. There are typically three main energy loss mechanisms: Ohmic loss of MMs, dielectric loss of the substrate, and radiation loss of MMs. The most common method to increase Q-factor of MMs without changing material properties is to design asymmetric split resonators (ASRs) by breaking the symmetry of the MMs. The asymmetric design reduces the radiation loss of the resonator and increases Q-factor from 3 up to 30. Another method uses coupling between MMs in a super unit to excite both odd and even modes of the MMs. This approach improves Q-factor by a factor of 5 compared to typical film-based MMs. However, the Q-factor of THz MMs needs to be further enhanced (10 to 20 times) to meet the requirement of ultra-sensitive sensors.
Another factor that measures the sensitivity of MM sensors is how much the resonant frequency shift in the transmission spectrum when permittivities of the adjacent medium change. Modern detection techniques require sensors to have the ability to detect a very small quantity of substances, even single molecules. However, it is extremely difficult to achieve such a high sensitivity using typical film-based MM sensors because the response to changes of substance, in the form of small resonant frequency changes, can be hard to detect, especially when the volume or concentration of the substance around the MMs is not high enough. In order to develop sensors that can detect minute concentration of substances, large resonant frequency change in response to the change of the substance around the MMs is one of the key requirements.